The present invention relates to catalytic combustion devices, and more specifically, to catalytic combustion devices that combust an anode effluent containing unused hydrogen (H2) and a cathode effluent containing an unused oxidant, such as oxygen (O2) or air, to produce heat.
Catalytic combustion devices are employed in a variety of applications. A typical application involves the use of the catalytic combustion device to combust left over fuels that are contained within effluents exhausted from a power system within which the catalytic combustion device is employed. The power systems within which the present invention can be employed use a fuel source, such as hydrogen (H2) and an oxidant source, such as oxygen (O2) and/or air (O2 admixed with nitrogen (N2)) to produce electrical power. The creation of electrical power within the power system results in effluents that are exhausted from the power system. The effluents typically contain unused fuel in the form of H2 and unused oxidant in the form of O2 and/or air. These effluents represent a source of energy that can be used. To extract the energy from the effluents, these power systems typically employ a catalytic combustion device that combusts the unused H2 contained within the effluent to produce heat that can be used within the power system to meet a heat demand.
The amount of H2 contained within the effluent will vary depending upon the efficiency of the power system and the conditions under which the power system is operated. Because the amount of H2 contained within the effluent varies, the catalytic combustion device typically includes a liquid fuel supply that can be used to increase the amount of combustible fuel within the combustion device so that heat demands placed on the combustion device by the power system can be met. Additionally, because the amount of H2 contained within the effluent varies, the amount of reaction occurring in any particular area within the combustion device can also vary and result in hot spots or locations of excessive heat that can damage the combustion device. The effluents and any liquid fuel flowing into the combustion device are a flammable fuel mixture. The temperature at which the fuel mixture will autoignite will vary depending upon the composition of the fuel mixture.
Conventional combustion devices are designed to preclude autoignition of the fuel mixture. When autoignition of the fuel mixture within some areas occurs, the combustion device typically is damaged and possibly completely destroyed. In one solution to the autoignition concern, the fuel mixture is passed through a high density foam structure, prior to entering the area of the combustion device where the catalytic reaction is occurring and excessive heat build up can occur. The high density foam structure induces mixing as well as producing a high velocity exit gas. As long as the velocity of the combustible fuel mixture exiting the high density foam structure is greater than the fuel mixture flame speed and the material is below the autoignition temperature, the fuel mixture upstream of the high density foam structure will not ignite. That is, the high density foam structure acts as a flame arrestor and prevents flame propagation across the high density foam structure.
While the use of the high density foam structure may prevent flame propagation to an undesirable area in the combustion device, the high density foam structure produces a significant pressure drop as the fuel mixture flows through the high density foam structure. The pressure drop is undesirable because it may require the effluents flowing into the combustion device to pass through additional equipment to increase the pressure of the effluents prior to entering the combustion device so that adequate pressure and flow of the effluents through the combustion device is achieved. The extra equipment to pressurize the fuel flow increases the complexity and cost of the system within which the combustion device is employed.
Therefore, it would be desirable to provide a combustion device that does not require the use of a flame arrestor or reduces the density of the flame arrestor so that the pressure drop across the flame arrestor is smaller and does not require the effluents to flow through any additional equipment prior to entering the combustion device. Additionally, it would be desirable to provide a combustion device that can utilize a liquid fuel injection system to provide a fuel to the combustion device so that the combustion device can meet a heat demand of the power system during the start up operation of the power system where the amount of effluent being exhausted by the power system may not be sufficient to meet the heat demand of the power system.
The present invention is directed to a catalytic combustion device that diminishes and/or eliminates the need for a flame arrestor within the combustion device. This is accomplished by splitting the H2 containing effluent exhausted by the power system into a plurality of flows and injecting the plurality of flows in multiple locations along the combustion device. The injection of the flows are controlled so that the fuel mixture within the combustion device is at a concentration that has an autoignition temperature that is above the operating temperature of the various sections of the combustion device. The present invention also provides a method of operating such a combustion device. The invention further discloses a method of starting up the combustion device when the flow of H2 within the anode effluent is not sufficient to meet the heat demand placed on the combustion device.
The catalytic combustion device of the present invention comprises a first section that receives an oxidant feed stream and a first portion of an anode effluent stream. The oxidant feed stream and the first portion of the anode effluent stream mix together in the first section to form a first stage flow stream. There is a second section downstream from the first section. The second section has a first catalyst bed. The second section receives the first stage flow stream from the first section and directs the first stage flow stream through the first catalyst bed. There is a third section downstream from the second section. The third section receives the first stage flow stream from the second section. The third section also receives a second portion of the anode effluent stream. The first stage flow stream mixes with the second portion of the anode effluent stream in the third section to form a second stage flow stream. There is a fourth section downstream from the third section. The fourth section has a second catalyst bed. The fourth section receives the second stage flow stream from the third section and directs the second stage flow stream through the second catalyst bed.
The present invention discloses a method of operating a catalytic combustor that combusts a flow of an anode effluent. The method includes the steps of: 1) proportioning an anode effluent flow into a plurality of portions; 2) supplying a first portion of the anode effluent flow to a first stage of the combustor; 3) supplying an oxidant feed stream to the first stage of the combustor; 4) mixing the first portion of the anode effluent flow and the oxidant feed stream in the first stage of the combustor to form a first stage flow; 5) reacting the first stage flow within a first catalyst bed as the first stage flow passes through the first catalyst bed; 6) passing the first stage flow to a second stage of the combustor that is downstream of the first stage; 7) supplying a second portion of the anode flow to the second stage of the combustor; 8) mixing the second portion of the anode flow with the first stage flow within the second stage of the combustor to form a second stage flow; and 9) reacting the second stage flow within a second catalyst bed as the second stage flow passes through the second catalyst bed.
The present invention also discloses a method of starting a catalytic process within a catalytic combustor with a liquid fuel until a sufficient flow of anode effluent is available. The method includes the steps of: 1) supplying a liquid fuel flow to the combustor in a quantity sufficient to meet a heat demand of a known magnitude placed on the combustor; 2) supplying an oxidant feed stream to the combustor; 3) mixing the liquid fuel and oxidant feed stream together in the combustor to form a fuel/oxidant flow; 4) vaporizing the fuel/oxidant flow with a heating element within the combustor as the fuel/oxidant flow passes therethrough; 5) reacting the vaporized fuel/oxidant flow in a primary catalyst as the vaporized fuel/oxidant flow passes through the primary catalyst so that the combustor generates heat to meet the heat demand; 6) exhausting the reacted fuel/oxidant flow from the combustor; and 7) maintaining the supplying of the liquid fuel flow to the combustor until the combustor is supplied with an anode effluent flow of a magnitude capable of allowing the combustor to meet the heat demand without the liquid fuel flow.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.